1. Field of the Invention
This invention relates to coating protection of components or articles for power generating apparatus, and more particularly to the reduction of heat load on and resistance to surface degradation of components.
2. Discussion of the Prior Art
One type of power generating apparatus including a section operating at high temperatures is the gas turbine engine. As gas turbine engine technology advanced, desirable design operating conditions in the hot operating sections of the engine exceeded the temperature capability of the alloys from which engine components were manufactured. Generally, such hot operating components are manufactured from materials frequently referred to as high temperature superalloys primarily based on Ni or Co or both.
It is widely practiced in the art to include internal air cooling passages in the interior of the component, with surface connected air cooling discharge ports. This arrangement reduces heat load on the component and enables the design of outer surface film cooling. One example of an air cooled gas turbine engine component in the form of a blading member is in U.S. Pat. No. 5,503,527--Lee et al. (patented Apr. 2, 1996). However, use of engine air for air cooling is at the expense of and can reduce performance potential. It is desirable, therefore, to reduce some of the heat load on the component other than through the use of cooling air.
One source of heat energy experienced by a component such as in the turbine and exhaust sections of a gas turbine engine is radiant heat energy from an upstream combustor. Such energy is in a frequency range that can increase substrate temperature and heat load on the article, and decrease article life. A multi-layer coating applied to a substrate and which reflects such heat energy from the component surface has been reported to provide reduction in heat load on the article. Such a coating is described in U.S. Pat. No. 5,851,679--Stowell et al. (patented Dec. 22, 1998).
In addition to such thermal exposure and heat load problems on a gas turbine engine component is the effect of a strenuous gas stream environment on an article surface, generally in the temperature range of about 1400-2000.degree. F. Downstream of a combustor, the engine gas stream includes high temperature oxygen along with products of fuel combustion with corrosive and sulfidizing elements, all of which can adversely affect the surface of a component. Therefore, a combination evolved of air cooling to reduce to component temperature and heat load on the component, along with surface environmental protection from oxidation, corrosion and sulfidation, generally in the form of environmental resistant coatings. Engine operating experience has shown that direct exposure of the coatings to such strenuous operating conditions results in coating degradation, including oxidation, and requires periodic repair or replacement of such coatings
Environmental resistant coatings for gas turbine engines, many including Al, have been used and widely reported. These include U.S. Pat. No. 3,667,985--Levine et al (patented Jun. 6, 1972) relating to the commercially available Codep aluminide type of coating; U.S. Pat. No. 4,313,760--Dardi, et al (patented Feb. 2, 1982) relating to the commercial M--Cr--Al type of coating, in which M refers to at least one of Ni, Co, and Fe, and with which has been included additional elements, alone or in combination, such as Y, Hf, and Pt; and U.S. Pat. No. 5,658,614--Basta et al (patented Aug. 19, 1997) relating to the Pt--Al type of coating widely used in the gas turbine art.
In addition to the effect of temperature and chemical environment on gas turbine engine components during operation is the effect of abrasive particles carried in the engine gas stream and impinging on the component surface. Such impingement can lead to an increase in surface roughness and accumulation of deposits that can result in performance degradation and design limitations. For example, in one typical engine operation, the surface roughness of a turbine blade airfoil surface can change from about 50 micro-inch to over 200 micro-inch after about 4000 hours service. As a result of such degradation, the skin friction and aerodynamic drag increase. In addition, the heat transfer to the airfoil can double and metal surface temperature can increase over 100 degrees Fahrenheit. Such doubling of the heat load either significantly reduces life of the component or causes the designer of the component to design to meet the higher heat load with a higher cooling flow rate, thereby foregoing greater performance possibilities. Therefore, use of a protective dielectric coating has been proposed to reduce such particle deposits by reducing the magnitude of an electrostatic force that attracts particles flowing through the engine to the surface of a component. This has been discussed in co-pending patent application Ser. No. 09/191,824--Ackerman, et al., filed Nov. 13, 1998.
As was mentioned, after periods of operation in an engine, normal wear and deterioration of the environmental protective coating has been seen to occur. This is the result of contact directly with abrasive particles as well as with such elements that are oxidizing and sulfidizing in the products of combustion or with oxygen diffusing through an outer layer such as the above described dielectric coating. Provision of a coating that resists degradation from the combination of particle impingement and attachment, direct environmental attack, diffusion of adverse environmental elements through a coating system, and exposure to heat energy which can increase heat load on an article, can increase component life and allow a designer to improve engine performance.